Tyrosine kinases are thought to play a major role in the control of cell growth and differentiation. Most of the work on the phosphorylated product was performed, however, on isolated proteins or cultured cell lines. To assess the overall involvement of tyrosine phosphorylation in vivo, a monoclonal antibody (MAb) against phosphotyrosine was applied to conventional histological sections of various tissues. With the immunoperoxidase staining method, two unique patterns of intracellular distribution of phosphotyrosine were identified among a large variety of normal tissues. (a) In many of the epithelia examined, a peripheral staining was observed, either at the apical aspect alone or at the entire contact region between neighboring cells. This pattern of staining seems to coincide with the distribution of a subset of cytoskeletal elements and requires pre-treatment with proteinase K. (b) In most steroidogenic tissues examined, vesicular cytoplasmic staining was evident, which seems to represent steroid-containing granules. In this case, proteolytic pretreatment is not essential and can be harmful. An extensive survey of human ovarian carcinoma biopsies failed to reveal any consistent staining pattern. These findings might indicate the involvement of tyrosine phosphorylation in basic cellular activities such as the assembly of the specialized cytoskeletal components.